1. Field of the Invention
The present invention relates to a still-image taking camera and, more particularly, to a still-image taking device which is capable of taking still-images of a plurality of areas of which a portion of an object is divided and of synthesizing a plurality of still-images into a highly fine still-image.
2. Description of the Background Art
Generally, still-image taking cameras provided with solid-state image sensors have small size and low energy consumption and are broadly used in video still cameras, digital still cameras, image scanners, facsimiles and so on.
Solid-state image sensors for domestic use conventionally use about 400 thousand pixels for area sensing and about 2000 pixels for line sensing. Recently, to meet an increasing demand for obtaining high resolution images, there have been manufactured high-resolution image sensors of several million pixels for area sensing and of 5000 to 10000 pixels for line sensing.
Such high-resolution solid-state image sensors, however, are very expensive and can not be expected to be available at a reduced cost in the near future.
Japanese Laid-Open Patent No. 3-240372 and No. 5-260264 disclose a still-image taking camera which, instead of such an expensive image sensor, uses a relatively cheap image sensor for obtaining a high resolution image in such a manner that a still-image is taken from each of divided areas of which a specified portion of an object is divided and then a plurality of the still-images of the divided areas are synthesized to form a high-resolution still-image.
In a conventional still-image taking camera, an image input portion divides a specified portion of an object into a plurality of areas and takes a still-image of each area. A solid-state image sensor is installed on an X-Y-movable table or manually held and is moved in a two-dimensional plane when taking still images of the divided areas of the object. Still-images as taken are partially overlapped with each other for assuring easy synthesizing thereafter.
Divided-area images taken by the image input portion sequentially enter into a divided-image storage.
A synthesizing information extracting portion extracts information necessary for image synthesizing from each divided-area image stored in the divided-image storage. The synthesizing information may include various kinds of information, e.g., information on motion vectors between divided images, information for correcting image-to-image brightness, magnification ratios of multi-focused images, deformation correcting information for images taken at an upward angle and so on. The synthesizing information extracted by the synthesizing information extracting portion is transferred to an image synthesizing portion.
The image synthesizing portion generates an entire area image by synthesizing divided-area images stored in the divided-image storage according to their synthesizing information.
For instance, in a process of synthesizing divided-area images on the basis of synthesizing information on image-to-image motion vectors, an area of an object is divided into some sub-areas and respective still-images are taken as partially overlapped with each other and are then stored in the divided-image storage.
Image-to-image displacement values may not be constant because of movement error of the moving table or hands. Therefore, the divided images can not be matched with each other without using motion vectors extracted by the synthesizing information extracting portion. If the divided images are synthesized by the image synthesizing portion without using image-to-image motion vectors, an obtained entire area image lacks continuity and has separated lines between the synthesized divided images therein.
On the contrary, divided images are taken from respective divided-areas of an object portion in such a way that they may be partially overlapped with each other, and then a specified block (searchable block) is set in each overlapped portion of each divided-image by the synthesizing information portion. Motion vectors can be extracted from each divided image by detecting movement of searchable blocks. This method is a so called block matching method, by which divided images are well matched with each other. Namely, an entire area image of satisfied continuity can be reproduced by synthesizing the divided images by the image synthesizing portion using the matching method.
The entire area image generated by the image synthesizing device is transferred and stored in the synthesized image storage. This image may be outputted by the synthesized image output portion directly to a printer or a display unit if necessary. It may be compressed to be recorded in a memory card or the like.
In the conventional still-image taking camera, divided images taken by the image input portion are stored in the divided image storage which therefore must have a large memory capacity and may be very expensive. Furthermore, the image synthesized by the image synthesizing portion as obtained is stored in the synthesized image storage which, therefore, must have a large memory capacity and may also be very expensive.
For example, to obtain a highly fine image of A4 size (JIS: Japanese Industrial Standards), it is needed to use a divided-image storage having a capacity of 16 Megabytes (MB) to store only divided-area images and practically having a capacity of 24 MB to store divided-area images plus overlaps for matching images.
A storage having such large memory capacity may be very expensive even if it is constructed as a DRAM type which is currently available at lowest cost. Such a storage is, therefore, unsuitable for use in home-electronic devices. The use of large capacity memory may also hinder the development of goods which are more miniaturized and consume less electric energy. Goods with a large capacity memory may thus be limited in its field of application.
To solve the above-mentioned problems, a still-image taking camera may be so modified that an image compressing unit is provided between the image input portion and the divided image storage so as to compress each divided image input by the image input portion and then store the compressed image in the divided image storage and to compress the entire area image synthesized by the image synthesizing portion and then store the compressed synthesized image in the synthesized image storage. This modification may reduce the required capacity of the divided image storage and the required capacity of the synthesized image storage.
However, each divided image, if simply compressed, may have uneven compression even in one line and, therefore, may not allow the extraction of information necessary for synthesizing it together with other compressed divided images. Consequently, the compressed divided images may not correctly be synthesized by the image synthesizing portion.
Accordingly, the compressed divided-area images stored in the divided image storage are extended into their original state allowing extraction of synthesizing information therefrom, then they are synthesized into an entire area image according to the extracted synthesizing information.
This method, however, requires that every divided-area image must be extracting entirely for extending therefrom necessary synthesizing information and an entire-area image synthesized from the extended divided-area images must be compressed again to be stored in the synthesized image storage if its memory capacity shall be reduced. These time-consuming operations much decrease the effectiveness of real-time image processing.